15 and my friend's sending dirty pictures?

[Alicia]

My friend Rose showed me the messages on Facebook between her boyfriend and herself and they're really dirty! She let me flick through some of them and when she wasn't looking I looked through the other ones and they're really inappropriate for a 15 year old haha. I know it was so sneaky of me to look through the rest of the messages but what I saw was horrible. They talked about her being wet, about how they want to be together so they can have sex all night, she said 'I want you inside me' and then they were sending each other really dirty pictures and I was almost sick! I flicked past the pictures as I wasn't too keen on seeing my best friend naked haha. Anyway, should I tell her I saw the messages? Not only that, but she's also messaging another lad saying she loves him and is so glad they had spent the night together and had sex. She told the guy she was single when she isn't... I feel disgusted at myself for invading her privacy and her for doing that! Yeah, everyone does things they're not proud of but she's so young to be doing all of this.... She never acts like this normally and I don't know what's happened to her. What should I do?